Super-resolved polarisation-enhanced second harmonic generation for direct imaging of nanoscale changes in collagen architecture

Super-resolution (SR) optical microscopy has allowed the investigation of many biological structures below the diffraction limit, however, most of the techniques are hampered by the need for fluorescent labels. Non-linear label-free techniques such as Second Harmonic Generation (SHG) provide structurally specific contrast without the addition of exogenous labels, allowing observation of unperturbed biological systems. We use the photonic nanojet (PNJ) phenomena to achieve super-resolution SHG (SR-SHG). A resolution of ~λ/6 with respect to the fundamental wavelength, that is, a ~2.3-fold improvement over conventional or diffraction-limited SHG under the same imaging conditions is achieved. Crucially we find that the polarisation properties of excitation are maintained in a PNJ. This is observed in experiment and simulations. This may have widespread implications to increase sensitivity by detection of polarisation-resolved SHG (p-SHG) by observing anisotropy in signals. These new findings allowed us to visualise biological SHG-active structures such as collagen at an unprecedented and previously unresolvable spatial scale. Moreover, we demonstrate that the use of an array of self-assembled high-index spheres overcomes the issue of a limited field of view for such a method, allowing PNJ-assisted SR-SHG to be used over a large area. Dysregulation of collagen at the nanoscale occurs in many diseases and is an underlying cause in diseases such as lung fibrosis. Here we demonstrate that pSR-SHG allows unprecedented observation of changes at the nanoscale that are invisible by conventional diffraction-limited SHG imaging. The ability to non-destructively image SHG-active biological structures without labels at the nanoscale with a relatively simple optical method heralds the promise of a new tool to understand biological phenomena and drive drug discovery.